Liquid substances applied to the airfoil surfaces of aircraft to prevent their freezing and being covered with ice in inclement weather are well-known and are important to ensure safe and proper take off of aircraft in winter. It is also well-known that aircraft departures are often delayed under such conditions and the anti-icing formulations must often be reapplied if the plane must wait an extended period. These liquids must also be stable not only through temperature extremes, but also during pumping and spraying (application to the wings) and the taxiing phases of the take off procedure and also must adhere to the wing surfaces during taxiing and ground winds. The fluids may also be diluted somewhat through intentional dilution with water, such as during particularly cold conditions below the expected operating temperatures of the fluids to enable their application, or incidentally as through freezing rain conditions. In addition to these demands, the anti-icing materials must also change characteristics, becoming much less viscous, and flow off the wing surfaces during the relatively high shear conditions of take off. Such compositions are also useful in the de-icing or anti-icing of other surfaces besides air foils, for example, runway and vehicle roadway surfaces.
Other anti-icing compositions are known. For example, U.S. Pat. No. 4,744,913 describes an anti-icing and de-icing agent, based on glycols and water and having crosslinked acrylic polymers useful as a thickener, and also containing customary corrosion inhibitors, surfactants belonging to the group of alkali metal alkylarylsulfonates and a neutralizing agent to adjust the pH to a basic value. The agents contains, as the thickeners, two selected crosslinked acrylic polymers in a specific ratio by weight to one another, namely a selected crosslinked acrylic acid or alkali metal acrylate homopolymer and a selected crosslinked acrylic acid/acrylamide or alkali metal acrylate/acrylamide copolymer in a ratio by weight of 2:1 to 10:1. The neutralizing agent may be three compounds, each in a selected amount, namely ammonia, monoethanolamine, diethanolamine and/or triethanolamine as the first alkaline compound, potassium hydroxide as the second alkaline compound and a further alkali metal hydroxide as the third alkaline compound. The new agent was found to exhibit the particular advantage that it has a relatively low viscosity even at arctic temperatures and low shear rates, which ensures rapid and complete runoff of the agent at the take-off of the aircraft even under extreme conditions.
Another liquid agent for de-icing and protecting against icing-up is described in U.S. Pat. No. 4,358,389, by means of which it is possible, in particular, to free the metal surface of aircraft rapidly and completely from ice hoar-frost, snow and the like, and to protect the surface against further build-up for a relatively long period. The agent is essentially composed of several components, namely of (a) glycols, (b) water, (c) thickeners, (d) substances insoluble in water, (e) surface-active agents, (f) corrosion inhibitors and (g) alkaline compounds. The quantities are very specific in each case, the quantity of the components (a) and (b) being at least 94%, relative to the total weight of the agent. Its pH is 7.5 to 10. The component (c) thickeners are crosslinked polyacrylates described in the patent in further detail and also in U.S. Pat. No. 2,923,692.
U.S. Pat. No. 4,954,279 describes a composition consisting of a microemulsion of oil in a water/glycol solution containing certain thickening agents (e.g. natural and artificial gums, cellulose ethers, carboxymethylcellulose and hydroxyethylcellulose), emulsifiers, substantially water-insoluble, partially polar oils, and alkanolamines to provide the desirable viscosity and shear stability needed to give a composition having effective de-icer and anti-icing properties. The emulsifier may be anionic surfactants, nonionic surfactants and mixtures thereof, such as organo-phosphates, phosphonates, sulfates, sulfonates, fatty acid salts, alcohols, phenols, amines, fatty acids and their alkoxylated derivatives.
A composition for accelerating the melting of layers of snow or ice using anionic, nonionic and cationic wetting agents in the absence of other melt-accelerating agents is described in U.S. Pat. No. 3,412,030. The anionic wetting agent may be an alkyl aryl sulfonate, having at least one alkyl group having 12 to 15 carbon atoms. The nonionic wetting agent is selected from the group consisting of alkyl oxyethers and esters and thioethers and esters having the formula: EQU Ra(CH.sub.2 CH.sub.2 O).sub.x --CH.sub.2 CH.sub.2 OH
where A is selected from the group consisting of ethereal oxygen and sulfur, amino, carboxylic ester and thiocarboxylic ester groups, R is a saturated or unsaturated hydrocarbon group or an aralkyl group of 8 to 18 carbon atoms and x is from 1-20.
British Patent Specification 1,272,464 discusses a deicing fluid having an aqueous solution of an alcohol component which comprises one or more of ethylene glycol, propylene glycol and glycerol; a polyethylene glycol ether of a diisoalkylphenol; and a thickening agent which is a copolymer of acrylic acid or methacrylic acid.
Anti-icing compositions thickened using a blend of a polyacrylic acid and a copolymer of polyacrylic acid with a vinyl monomer, where the copolymer acts as a thickener modifier, similar to that used in the present invention is described in U.S. Pat. No. 5,118,435 to Edward C. Nieh of Texaco Chemical Company, incorporated by reference herein. The two polymers form a thickener blend. The polyacrylic acid may have a weight average molecular weight of from about 2,500,000 to about 3,500,000 and be slightly crosslinked with a diene monomer. The compositions also contain an alkylene glycol component, such as propylene glycol, diethylene glycol and mixtures thereof. These compositions retain a high viscosity, even when diluted with water, and exhibit a highly pseudoplastic rheology indicating desirable flow off characteristics. A neutralizing agent to neutralize at least partially the carboxylic acid groups of the thickening blend is also preferably included.
There remains a need for thickened aircraft anti-icing compositions that exhibit all the rheological properties that yield the performance characteristics described in the first paragraph of the Background of the Invention herein. It would also be advantageous if the fluid retained a high viscosity as the composition is diluted with water. The anti-icing fluids should also remain stable during shear due to pumping and/or spraying, and should be physically stable on a surface such as an airfoil for relatively long periods of time to delay or avoid re-application of the compositions during departure holdovers, particularly during freezing rain, or generally humid and cold conditions, for example.